Selenoglutathione (GSeH) is a water-soluble tripeptide, in which the sulfur atom of biologically important reductant glutathione (GSH) is replaced by a selenium atom, and exhibits a higher reducing activity than GSH. In this review, we overview the research on this selenium analogue of glutathione and look ahead to future research directions. Both solid-phase and liquid-phase methods can be used to chemically synthesize selenoglutathione diselenide (GSeSeG), the oxidized form of GSeH. Lesser amounts of GSeH are also found in the metabolic products of yeast and certain plants (garlic, sunflower sprouts, etc.) grown in a high-selenium medium. In the meantime, a biological method for the synthesis of GSeH using mutated yeast has recently been reported. Various applications of selenoglutathione in the field of biochemistry have already been explored. It has been reported that GSeSeG is an efficient catalyst for the oxidative folding of proteins. GSeSeG is also an excellent radical scavenger and potential detoxifier of intracellular xenobiotics. Recently, it has also been reported that GSeSeG has stress-reducing and antibacterial properties. Because GSeSeG has low toxicity, its unique reactivity is expected to be widely applied in the fields of applied biology and medicine.

Selenoproteins, the functional form of the essential trace element selenium, play a vital role in maintaining redox homeostasis in humans. Selenocysteine (Sec), which constitutes the active center of many selenoproteins, is introduced to the polypeptide chain by a unique biosynthetic insertion mechanism, making the expression of selenoproteins through biological means challenging. Compared to its analogue cysteine (Cys), Sec exhibits a lower redox potential, facilitating the oxidation of selenol groups to form diselenide bonds. These diselenide bonds are more resistant to reduction than disulfide bonds, providing an enhanced stability to peptides under reducing conditions. On the other hand, due to the larger atomic radius of selenium, the dissociation energy of the diselenide bond is lower than that of the disulfide bond, rendering them more prone to diselenide metathesis. This mini-review summarizes the use of Sec for the chemical synthesis of proteins, Sec-containing peptides and the selenoproteins. The diselenide metathesis reaction of Sec-containing peptides is also reviewed.

Selenoprotein P (SeP) is the major selenium transport protein in the blood and plays a central role in selenium metabolism by being involved in selenoprotein synthesis via selenium supply in various tissues. On the other hand, excess selenoprotein P, which is increased in patients with diabetes and other diseases, can be a malignant protein that causes metabolic disorders in various tissues through disruption of redox homeostasis. Therefore, developing methods to control selenium metabolism in physiological and pathological conditions are significant. In this study, we focused on epigallocatechin gallate (EGCg), an electrophilic plant component, and newly found that modification of the cysteine residue in SeP by this molecule inhibits its cellular uptake in SH-SY5Ycells. SeP-EGCg adduct failed to induce the expression of glutathione peroxidase, which is synthesized in cells by selenium supply through SeP. These results suggest that EGCg can be a candidate molecule to induce negative remodeling of selenium metabolism by inhibiting SeP incorporation into the cells.

The formation of selenium (Se) adducts of protein in the liver of rats administered Se in excess of nutritional requirements was confirmed using high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICPMS). Male Wistar rats aged 4 weeks old were divided into three groups and fed a basal Se-deficient diet or basal diet supplemented with 0.2 or 2.0 µg Se/g of selenite for 4 weeks, respectively. The liver Se concentration and GPX activity were markedly elevated in rats fed the Se-added diet; the 2.0 µg Se/g group showed a higher Se concentration than the 0.2 µg Se/g group, but GPX did not differ between the two groups. HPLC-ICPMS analysis of liver protease hydrolysates led to the detection of only selenocystine in the 0.2 µg Se/g group, while the 2.0 µg Se/g group showed the presence of four unknown Se compounds in addition of selenocystine. In another experiment, rats weighing 250 g and previously fed the Se-deficient diet for 4 weeks were intraperitoneally administered 50 µg Se/day of selenite or L-selenomethionine for 7 days, and their liver protease hydrolysates were analyzed by HPLC-ICPMS. In selenite-treated rats, peaks of several unknown Se compounds other than selenocystine were detected. In selenomethionine-treated rats, selenomethionine was detected in addition to selenocystine. Unknown Se compounds were also present, but the number and height of peaks were smaller than in selenite-treated rats. These results indicate that with supranutritional Se, accumulation in organs occurs in the form of Se adducts on selenite exposure and mainly nonspecific insertion of selenomethionine into positions of methionine residues of proteins on selenomethionine exposure.

Selenium (Se) was recently shown to be beneficial for plants, and its application to crop production, drug development, and environmental hygiene is expected. The enzyme activities of some proteins were previously reported to be enhanced in plants grown with Se. These increases may be attributed to the binding of Se to catalytic cysteine by post-translational modifications (PTMs), which is a novel mechanism of expression. Therefore, the present study investigated Se binding and increases in enzyme activity by PTMs to cytoplasmic glyceraldehyde-3-phosphate dehydrogenase (GAPC) from broccoli (B. oleracea var. italica), which is classified as a Se-accumulating species, in planta and in vitro. GAPC derived from plants cultivated with 1 µM selenate had more Se bonds and stronger enzyme activity than that from those cultivated without selenate. BES-Thio, a fluorescent probe that identifies thiol or selenol groups, revealed that increases in GAPC activity by Se binding were due to the formation of a selenol group, which is more reactive than a thiol group, on GAPC-catalyzed cysteine by PTMs. Furthermore, purified recombinant GAPC and mutants (C156S, C160S, and C156S/C160S) were reduced and reacted with GSSeSG in vitro to investigate Se binding and selenol group generation by PTMs to recombinant GAPC and mutant proteins. The results obtained show that Se binding and selenol group generation by PTMs occurred only at Cys 156, which corresponds to the catalytic Cys. In addition, V_max, K_cat, and K_cat /K_m values of Se binding GAPC synthesized in vitro using purified BoGAPC were 1.49-, 1.48-, and 1.86-fold higher, respectively, than those of GAPC. These results indicate that the generation of selenol group at the catalytic Cys of GAPC by PTM improves the enzyme activity.

Selenophosphate synthetase SelD from Haemophilus influenzae (HinSelD) has a selenocysteine (Sec) residue at its active site, while its Escherichia coli homolog (EcoSelD), which shares 65% amino acid sequence identity, contains cysteine (Cys) instead. This difference prompts questions about the evolutionary divergence between Sec-type and Cys-type SelD enzymes. We used bioinformatics tools to compare the selenocysteine insertion sequence (SECIS) elements of the Sec-type selD gene with the corresponding sequence regions of the Cys-type genes. Our analysis showed vital conservation between the UGA Sec codon and SECIS secondary structures. We also tested if HinSelD SECIS could support Sec insertion in E. coli. Results indicated that HinSelD SECIS is recognized by E. coli, enabling Sec incorporation. Nucleotide differences between HinSelD and EcoSelD SECIS regions affected translation efficiency, with mutants G69A, A75G, G77A, and U84C showing 93%, 81%, 69%, and 69% of wild-type translation levels, respectively. Additionally, the Sec16Cys mutant of HinSelD exhibited a similar expression level compared to the wild-type, suggesting the secondary structure of the SECIS does not inhibit the translation of the preceding UGC codon for Cys.

Selenium (Se) is a rare metal refined from the slime byproduct of copper anodes. Selenium circulates globally in various valence states and forms. Soluble selenooxyanions, such as selenate (SeO₄^2-) and selenite (SeO₃^2-) are converted to volatile dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe) and vaporized. Although some microorganisms synthesize volatile Se compounds, and volatile Se compounds is used for resource recovery and soil remediation, the synthesis pathway of DMDSe has not yet been identified. We hypothesized that a methyltransferase in the Stutzerimonas stutzeri NT-I specific contig is involved in the synthesis of methylated Se in S. stutzeri NT-I and cloned the gene encoding the enzyme. We carried out qualitative analysis of synthesized volatile Se compounds using the transgenic E. coli DH5α pGEM-mdsN. A novel gene involved in DMDSe synthesis was identified and named mdsN and found to encode a class I SAM-dependent methyltransferase. When the mdsN was introduced into E. coli DH5α, the recombinant E. coli DH5α pGEM-mdsN acquired the ability to synthesize DMDSe, which corresponds to 62% of the initial Se concentration. In this paper, we report a novel finding that E. coli DH5α pGEM-mdsN, in which mdsN from S. stutzeri NT-I was transformed into E. coli DH5α, synthesized DMDSe from SeO₃^2- and Bio-Se⁰ from S. stutzeri NT-I.